‘The efficiency of water management systems is at the forefront of every new development and many retrofit developments.’
Technology can be transformative, as it was when Blackberry introduced the first generation “Smartphone” that allowed email to be used on a handheld device. But since then, each new development has been evolutionary, as handheld devices became accepted to the point where they are now a business essential that we think little about. However, Blackberry, who once defined this market, is now reduced to a historical footnote on the product development path.
We can think of stormwater management in much the same way as smartphone technology. When the concept of underground water storage was introduced as a commercial solution, this was transformative. Now, large catchment areas such as parking lots have their water controlled in an effective way, so much so that we give little thought to the inconvenience of surface water as it is quickly dispatched to the nether world of the sewer system.
Much the same way as we view the inconvenience of a malfunctioning app on a phone, we now view excess surface water the same way, as it is simply not encountered often enough to seriously think about it.
Today, with an increased focus on stormwater overwhelming storm sewer systems, governments have changed building codes to push this responsibility back on property owners, builders, developers and, ultimately, on architects and engineers. The efficiency of water management systems is at the forefront of every new development and many retrofit developments, as the increased stormwater volumes need to be managed in an efficient manner.
Ponds, cisterns, and arch and crate systems offer benefits for different applications and should be considered accordingly. Concerns about geology, topography, loading and historical events are all important deciding factors when choosing the most cost-effective one.
Ponds have been used for centuries. They are relatively low cost to construct and have a certain bucolic esthetic that can be used if the space required is available. Ponds can add to park-like settings which communities can be built around.
Unfortunately, this is where the benefits stop, mostly due to high land costs. Ponds are an inefficient use of land mass that developers loathe to give up for loss of revenue generating property. Significant risks in terms of public safety from drowning or unsafe outdoor skating rinks raise additional community concerns.
As witnessed in a few high-profile cases in western Canada, the impact of non-native invasive species such as coy fish and reptiles can have an impact on local environments, with many direct and indirect implications. Infrequent but costly maintenance is also a factor when considering ponds as overgrowth and silt buildup from airborne dust and debris can reduce the effective volume. Lastly, the holding of water for release to storm systems presents issues with solar warming that causes further issues downstream. Ponds may also become an undesired mosquito breeding ground.
Cisterns have been used for centuries to collect stormwater for reuse in agriculture and grey water applications. This practice has seen a recent resurgence, in an attempt to reduce dependence on municipal treated water. However, unless this is incorporated as part of a new development design, the high cost of retrofit for existing developments precludes this from being a mainstream application.
Other than for large infrastructure municipal applications, extreme landscaping depth limitations or significant volume requirements, the high initial costs limit this method’s viability for all but a few select applications that can support those costs and the associated civil engineering that accompanies it.
Prefabricated arch systems were a significant evolution in suburban and urban water management programs as the water management tank could be buried, similar to a cistern, but with a cost closer to a pond style system, allowing for the best of both worlds. The low initial cost combined with quick installation, limited equipment and civil involvement is why this is the standard for stormwater management for parking lots in many North American cities. It is also a common application for large new commercial developments where roof water must be mitigated.
As accepted as arch systems are, they face challenges for contractors and engineers due to the large excavation area they consume, large foot print, the limited configurations, and the relatively low void space of expensive clear crushed stone required to make up the tank volume. This restricts the ability to be used in a retrofit application or in urban environments where space is at a premium.
The arch system performs well in an infiltration application and can be used as a detention tank, but the complexity of sealing the arch and stone combination limits this application. Lastly, the reduced ability for inspection and maintenance limits the system to basic functionality.
The next evolution of stormwater tanks is the crate style system. This method combines all of the functionality of an arch system with a high void compact structure, that is ideal for both new development as well as retrofit applications. These plastic molded elements feature high strength to weight ratio for ease of construction with no civil requirement due to the pre-engineered nature of the product.
The ACO StormBrixx® crate system was developed in Europe to deal with centuries old stormwater issues. It offers efficient use of underground space with extreme void ratios of up to 97%. This high void ratio limits excavation costs and also eliminates the need for expensive clear crushed stone as this does not make up part of the tank volume.
Its modular “building block” nature allows for irregular shape tanks to be formed that are ideal for geographically challenged retrofit applications. Being modular also allows for smaller tanks to be placed closer to source points of stormwater, further reducing underground piping and manifolds to create simple and cost-efficient infiltration or detention tanks. Lastly, the open interior “grid” type architecture of these systems and the purposely designed inside “boulevards” allows for multiple port options and complete access for inspection and maintenance.
Furthermore, the design life span of theses systems is 50 – 60 years, making them long-term solutions for their intended applications.
The high initial cost of crate systems is often seen as a downside, especially when compared to arch-style systems. However, reduced excavation, limited stone requirement and extremely fast installation times can mean crate systems actually offer the lowest installed “cost per cubic metre” of storage.
The benefits of a crate style system were demonstrated at a recent project in Milton, Ontario, when site plan approvals of a subdivision were submitted. The town required the stormwater tank to be moved further away from existing dwellings. This led to a challenge for the designer, as they had to overcome the site constraints where this tank could be located. Traditional stormwater tanks would not be able to satisfy the volumes required in the given space.
ACO StormBrixx was selected as it was able to satisfy the space/volume requirement. The contractors gained approval for its use, as an irregular shaped tank would fit within the available space, providing a cost-effective and much quicker installation than traditional systems.
The advantages of the open cell structure further satisfied requirements for inspection and cleaning. To make use of this advantage, 12 StormBrixx inspection ports were placed around the tank to provide access for future inspections and maintenance, ensuring trouble-free operations for years to come.
Other factors to be taken into account were the type, size and frequency of static or live loads, type of top cover and backfill material, underground water levels, depth of underground infrastructure, and surface deflections under design live load.
Surface deflection is a particular design consideration, even though the specified tank may not collapse under the design load. Excessive deflections can lead to deterioration of the paved surface on top of the tank. This becomes quickly apparent, especially in cold climate areas as the seasonal temperature cycle will accelerate surface failures.
This article appears in ES&E Magazine’s April 2019 issue.